Various types of control systems--usually hydraulic--of agricultural implements are known; for example, one such control system which determines the depth of a plow within the soil is described in the journal "Landtechnik" ("Agricultural Technology"), 1974, page 151. Plow control systems generally are provided to maintain the depth of bite of the plough in the soil at a predetermined level which depends on the pulling force supplied by the tractor which pulls the plow. The depth of position of the plow is controlled, as described, with reference to the position of the tractor. It is, of course, also possible to maintain a certain depth with respect to a sensed average soil level. If the soil consistency changes, for example from heavy soil to light soil, the automatic positioning system will control the plow to be dropped, since the pulling force has decreased. The situation may occur that the plow is dropped too far, so that the tractor is no longer capable to pull the plow. The wheels of the tractor will slip, and spin. Forward movement of the tractor is no longer possible upon spinning of the wheels. Consequently, the pulling force decreases. This, again, is sensed by the automatic control system and, in accordance with decreased pulling force, causes the plow to be placed at a still deeper position. The effect of this operation is that the plow will get stuck in the position since, upon spinning of the wheels, or slippage, the plow has the tendency to drop.
It has previously been proposed--see German Patent Disclosure Document DE-OS No. 24 29 594--to provide a load control system for wheeled vehicles which can recognize slippage or spinning of wheels and can provide suitable countervailing control measures. This arrangement has the disadvantage that it requires additional transducers with respect to the driven and the freely running wheels. These transducers not only are expensive but, further, are generally not suitable for use in agricultural equipment since the rough and generally contaminated conditions in agricultural equipment cause malfunction, and particularly erroneous output pulses. While it is possible to provide shields and the like to prevent contamination by dirt, such additional arrangements, again, are subject to contamination and soiling, require frequent cleaning, and generally are unsuited for practical agricultural use.